1. Field of the Invention
This invention relates generally to wave propagating structures and is concerned more particularly with a travelling wave tube having a coupled cavity type of slow wave structure provided with improved heat dissipating means.
2. Discussion of the Prior Art
A travelling wave tube usually includes an elongated envelope having in one end portion thereof an electron gun disposed to direct an electron beam axially through a slow wave structure to a collector electrode in the other end portion of the envelope. A slow wave structure of the coupled cavity type generally comprises a conductive conduit having therein a linear array of transverse partition plates axially spaced apart to provide a sequential series of interposed cavities. The partition plates usually are provided with respective central apertures which are axially aligned with one another to permit passage of the electron beam sequentially through the cavities to the collector electrode.
The tube envelope generally is provided with suitably spaced input and output means for introducing radio frequency (RF) wave energy into a cavity adjacent the electron gun, and for extracting corresponding amplified wave energy from a cavity adjacent the collector electrode. The RF wave is propagated through the slow wave structure by way of respective cavity coupling slots eccentrically disposed in the partition plates and radially spaced from the respective central apertures therein. The cavity coupling slots may be arranged in staggered relationship by disposing adjacent slots on opposing sides of the structural centerline to provide a folded waveguide type of propagating structure. As a result, the RF wave is propagated along the slow wave structure with an effective axial velocity comparable to the velocities of the electrons in the axially directed beam. Consequently, in successive cavities of the structure, the RF wave interacts with electrons in the beam to obtain therefrom a net transfer of energy whereby the RF wave is amplified.
It is well known that the propagation bandwidth of the described slow wave structure is determined by the sizes of the coupling cavity slots in the respective partition plates. Thus, for broad bandwidth amplification, the coupling cavity slots generally are provided with correspondingly large sizes by disposing them accurately in spaced concentric relationship with the respective central apertures in the plates. As a result, each of the cavity coupling slots is separated from the associated central aperture by an interposed arcuate web of plate material having respective ends integrally joined to the remaining larger portion of the plate. Since the inner surface of the arcuate web constitutes a wall portion of the central aperture, it is subjected to electron bombardment and rises to a prohibitively high temperature with increasing power requirements of the tube. Also, due to its restricted cross-section, the arcuate web cannot readily conduct the excess heat generated therein to the larger portion of the partition plate. Thus, due to its poor heat dissipation properties, the arcuate web of plate material is a limiting factor in the power handling capability of the tube.
Therefore, it is necessary and desirable to provide a travelling wave tube of the coupled cavity type with improved heat dissipating means for enhancing the power handling capacity of the tube.